Radio astronomy

Karl G. Jansky Very Large Array, a radio interferometer in New Mexico, United States

Radio astronomy is a subfield of

, was made through radio astronomy.

Radio astronomy is conducted using large radio antennas referred to as radio telescopes, that are either used singularly, or with multiple linked telescopes utilizing the techniques of radio interferometry and aperture synthesis. The use of interferometry allows radio astronomy to achieve high angular resolution, as the resolving power of an interferometer is set by the distance between its components, rather than the size of its components.

Radio astronomy differs from radar astronomy in that the former is a passive observation (i.e., receiving only) and the latter an active one (transmitting and receiving).

History

Holmdel, New Jersey, the world's first radio telescope, which was used to discover radio emissions from the Milky Way

Before Jansky observed the Milky Way in the 1930s, physicists speculated that radio waves could be observed from astronomical sources. In the 1860s, James Clerk Maxwell's equations had shown that electromagnetic radiation is associated with electricity and magnetism, and could exist at any wavelength. Several attempts were made to detect radio emission from the Sun including an experiment by German astrophysicists Johannes Wilsing and Julius Scheiner in 1896 and a centimeter wave radiation apparatus set up by Oliver Lodge between 1897 and 1900. These attempts were unable to detect any emission due to technical limitations of the instruments. The discovery of the radio reflecting ionosphere in 1902, led physicists to conclude that the layer would bounce any astronomical radio transmission back into space, making them undetectable.^[1]

Jansky announced his discovery at a meeting in Washington, D.C., in April 1933 and the field of radio astronomy was born.

After 1935, Jansky wanted to investigate the radio waves from the Milky Way in further detail, but Bell Labs reassigned him to another project, so he did no further work in the field of astronomy. His pioneering efforts in the field of radio astronomy have been recognized by the naming of the fundamental unit of

Grote Reber was inspired by Jansky's work, and built a parabolic radio telescope 9m in diameter in his backyard in 1937. He began by repeating Jansky's observations, and then conducted the first sky survey in the radio frequencies.^[12] On February 27, 1942, James Stanley Hey, a British Army research officer, made the first detection of radio waves emitted by the Sun.^[13] Later that year George Clark Southworth,^[14] at Bell Labs like Jansky, also detected radiowaves from the Sun. Both researchers were bound by wartime security surrounding radar, so Reber, who was not, published his 1944 findings first.^[15] Several other people independently discovered solar radio waves, including E. Schott in Denmark^[16] and Elizabeth Alexander working on Norfolk Island.^{[17][18][19][20]}

At

, created a radiophysics group at the university where radio wave emissions from the Sun were observed and studied. This early research soon branched out into the observation of other celestial radio sources and interferometry techniques were pioneered to isolate the angular source of the detected emissions. Martin Ryle and Antony Hewish at the Cavendish Astrophysics Group developed the technique of Earth-rotation aperture synthesis. The radio astronomy group in Cambridge went on to found the Mullard Radio Astronomy Observatory near Cambridge in the 1950s. During the late 1960s and early 1970s, as computers (such as the Titan) became capable of handling the computationally intensive Fourier transform inversions required, they used aperture synthesis to create a 'One-Mile' and later a '5 km' effective aperture using the One-Mile and Ryle telescopes, respectively. They used the Cambridge Interferometer to map the radio sky, producing the Second (2C) and Third (3C) Cambridge Catalogues of Radio Sources.^[21]

Techniques

Radio astronomers use different techniques to observe objects in the radio spectrum. Instruments may simply be pointed at an energetic radio source to analyze its emission. To "image" a region of the sky in more detail, multiple overlapping scans can be recorded and pieced together in a mosaic image. The type of instrument used depends on the strength of the signal and the amount of detail needed.

Observations from the

. Because of this, many radio observatories are built at remote places.

Radio telescopes

Radio telescopes may need to be extremely large in order to receive signals with low

, whereas a radio telescope "dish" many times that size may, depending on the wavelength observed, only be able to resolve an object the size of the full moon (30 minutes of arc).

Radio interferometry

The difficulty in achieving high resolutions with single radio telescopes led to radio

had been demonstrated by numerous groups in Australia, Iran and the UK during World War II, who had observed interference fringes (the direct radar return radiation and the reflected signal from the sea) from incoming aircraft.

The Cambridge group of Ryle and Vonberg observed the Sun at 175 MHz for the first time in mid July 1946 with a Michelson interferometer consisting of two radio antennas with spacings of some tens of meters up to 240 meters. They showed that the radio radiation was smaller than 10

in the UK).

Modern

has 27 telescopes giving 351 independent baselines at once.

Very-long-baseline interferometry

Beginning in the 1970s, improvements in the stability of radio telescope receivers permitted telescopes from all over the world (and even in Earth orbit) to be combined to perform

are possible.

The pre-eminent VLBI arrays operating today are the Very Long Baseline Array (with telescopes located across North America) and the European VLBI Network (telescopes in Europe, China, South Africa and Puerto Rico). Each array usually operates separately, but occasional projects are observed together producing increased sensitivity. This is referred to as Global VLBI. There are also a VLBI networks, operating in Australia and New Zealand called the LBA (Long Baseline Array),^[22] and arrays in Japan, China and South Korea which observe together to form the East-Asian VLBI Network (EAVN).^[23]

Since its inception, recording data onto hard media was the only way to bring the data recorded at each telescope together for later correlation. However, the availability today of worldwide, high-bandwidth networks makes it possible to do VLBI in real time. This technique (referred to as e-VLBI) was originally pioneered in Japan, and more recently adopted in Australia and in Europe by the EVN (European VLBI Network) who perform an increasing number of scientific e-VLBI projects per year.^[24]

Astronomical sources

GCRT J1745-3009

.

Radio astronomy has led to substantial increases in astronomical knowledge, particularly with the discovery of several classes of new objects, including pulsars, quasars^[25] and radio galaxies. This is because radio astronomy allows us to see things that are not detectable in optical astronomy. Such objects represent some of the most extreme and energetic physical processes in the universe.

The

.

Other sources include:

• Sun
• Jupiter
• Sagittarius A, the Galactic Center of the Milky Way, with one portion Sagittarius A* thought to be a radio wave emitting supermassive black hole
• Active galactic nuclei and pulsars have jets of charged particles which emit synchrotron radiation
• Merging galaxy clusters often show diffuse radio emission^[26]
• Supernova remnants can also show diffuse radio emission; pulsars are a type of supernova remnant that shows highly synchronous emission.
• The
  blackbody
  radio/microwave emission

International regulation

Radio astronomy service (also: radio astronomy radiocommunication service) is, according to Article 1.58 of the

or celestial objects.

Frequency allocation

The allocation of radio frequencies is provided according to Article 5 of the ITU Radio Regulations (edition 2012).[28]

In order to improve harmonisation in spectrum utilisation, the majority of service-allocations stipulated in this document were incorporated in national Tables of Frequency Allocations and Utilisations which is with-in the responsibility of the appropriate national administration. The allocation might be primary, secondary, exclusive, and shared.

• primary allocation: is indicated by writing in capital letters (see example below)
• secondary allocation: is indicated by small letters
• exclusive or shared utilization: is within the responsibility of administrations

In line to the appropriate

the frequency bands are allocated (primary or secondary) to the radio astronomy service as follows.

Allocation to services
Region 1	Region 2	Region 3
13 360–13 410 kHz  FIXED       RADIO ASTRONOMY
25 550–25 650          RADIO ASTRONOMY
37.5–38.25 MHz  FIXED MOBILE Radio astronomy
322–328.6     FIXED MOBILE RADIO ASTRONOMY
406.1–410     FIXED MOBILE except aeronautical mobile RADIO ASTRONOMY
1 400–1 427   EARTH EXPLORATION-SATELLITE (passive) RADIO ASTRONOMY SPACE RESEARCH (passive)
1 610.6–1 613.8 MOBILE-SATELLITE (Earth-to-space) RADIO ASTRONOMY AERONAUTICAL RADIONAVIGATION	1 610.6–1 613.8 MOBILE-SATELLITE (Earth-to-space) RADIO ASTRONOMY AERONAUTICAL RADIONAVIGATION RADIODETERMINATION- SATELLITE (Earth-to-space)	1 610.6–1 613.8 MOBILE-SATELLITE (Earth-to-space) RADIO ASTRONOMY AERONAUTICAL RADIONAVIGATION Radiodetermination- satellite (Earth-to-space)
10.6–10.68 GHz   RADIO ASTRONOMY and other services
10.68–10.7           RADIO ASTRONOMY and other services
14.47–14.5           RADIO ASTRONOMY and other services
15.35–15.4           RADIO ASTRONOMY and other services
22.21–22.5           RADIO ASTRONOMY and other services
23.6–24                RADIO ASTRONOMY and other services
31.3–31.5             RADIO ASTRONOMY and other services

See also

• Atacama Large Millimeter Array
• Channel 37
• Gamma-ray astronomy
• Infrared astronomy
• Radar astronomy
• Time smearing
• X-ray astronomy
• Waves (Juno) (radio instrument on the Juno Jupiter orbiter)
• Radio Galaxy Zoo
• Würzburg radar#Post-war use in astronomy

References

Further reading

Journals

• Gart Westerhout (1972). "The early history of radio astronomy".
  S2CID 56034495
  .
• Hendrik Christoffel van de Hulst (1945). "Radiostraling uit het wereldruim. II. Herkomst der radiogolven". Nederlands Tijdschrift voor Natuurkunde (in Dutch). 11: 210–221.

Books

• Gerrit Verschuur The Invisible Universe: The Story of Radio Astronomy Springer 2015
• Bruno Bertotti (ed.), Modern Cosmology in Retrospect. Cambridge University Press 1990.
• James J. Condon, et al.: Essential Radio Astronomy. Princeton University Press, Princeton 2016,
  ISBN 9780691137797
  .
• Robin Michael Green, Spherical Astronomy. Cambridge University Press, 1985.
• Raymond Haynes, Roslynn Haynes, and Richard McGee, Explorers of the Southern Sky: A History of Australian Astronomy. Cambridge University Press 1996.
• J.S. Hey, The Evolution of Radio Astronomy. Neale Watson Academic, 1973.
• David L. Jauncey, Radio Astronomy and Cosmology. Springer 1977.
• Roger Clifton Jennison, Introduction to Radio Astronomy. 1967.
• Jobn D. Kraus, Martt; E. Tiuri, and Antti V. Räisänen, Radio Astronomy, 2nd ed, Cygnus-Quasar Books, 1986.
• Albrecht Krüger, Introduction to Solar Radio Astronomy and Radio Physics. Springer 1979.
• David P.D. Munns, A Single Sky: How an International Community Forged the Science of Radio Astronomy. Cambridge, MA: MIT Press, 2013.
• Allan A. Needell, Science, Cold War and American State: Lloyd V. Berkner and the Balance of Professional Ideals. Routledge, 2000.
• Joseph Lade Pawsey and Ronald Newbold Bracewell, Radio Astronomy. Clarendon Press, 1955.
• Kristen Rohlfs, Thomas L Wilson, Tools of Radio Astronomy. Springer 2003.
• D.T. Wilkinson and P.J.E. Peebles, Serendipitous Discoveries in Radio Astronomy. Green Bank, WV: National Radio Astronomy Observatory, 1983.
• Woodruff T. Sullivan III, The Early Years of Radio Astronomy: Reflections Fifty Years after Jansky's Discovery. Cambridge, England: Cambridge University Press, 1984.
• Woodruff T. Sullivan III, Cosmic Noise: A History of Early Radio Astronomy. Cambridge University Press, 2009.
• Woodruff T. Sullivan III, Classics in Radio Astronomy. Reidel Publishing Company, Dordrecht, 1982.

External links

Wikimedia Commons has media related to Radio astronomy.

• nrao.edu National Radio Astronomy Observatory
• The History of Radio Astronomy * Reber Radio Telescope – National Park Services
• Radio Telescope Developed – a brief history from NASA Goddard Space Flight Center
• Society of Amateur Radio Astronomers
• Visualization of Radio Telescope Data Using Google Earth
• UnwantedEmissions.com A general reference for radio spectrum allocations, including radio astronomy.
• Improving Radio Astronomy Images by Array Processing Archived 2011-04-04 at the Wayback Machine
• What is Radio Astronomy – Radioastrolab